1. Field
One embodiment of the invention relates to a display apparatus and a device and method for controlling the backlight provided in the display apparatus. More particularly, one embodiment of the invention relates to a display apparatus that can impart enhanced gradation to dark images at low power consumption.
2. Description of the Related Art
As is known, digital television broadcasting has been promoted in recent years. In Japan, for example, not only satellite digital broadcasting systems, such as broadcasting-satellite (BS) digital broadcasting and 110-communications satellite (CS) digital broadcasting, but also the ground digital broadcasting has come into service.
TV broadcast receivers of a new type have come into use. They incorporate a record/playback device that automatically records any broadcast program that accords with the genre and keyword the user has input. When the user selects any broadcast program thus recorded, the record/playback device plays back the broadcast program.
The record/playback device needs to comprise a storage capacity large enough to record a number of broadcast programs. It also needs to access and retrieve any recorded program selected, as fast as possible. In view of this, a hard disk drive (HDD) is used as record/playback device.
The digital television broadcast receivers recently provided have a large display device and can be connected to various peripheral devices. In addition, they are connected, in increasing numbers, to networks. To improve the quality of images the display device may display, techniques have been developed, which adjust not only video signals, but also the amount of light the backlight emits.
To adjust the amount of light, the power-supply voltage of the backlight may be changed. When the power-supply voltage is changed, the image displayed may be degraded. Particularly in a large liquid crystal display, changes in the power-supply voltage of the backlight greatly influence the image gradation. It is therefore required that the power-supply voltage should be carefully controlled.
Jpn. Pat. Appln. KOAKI Publications 2004-110050 and 2005-148709 disclose the technique of controlling the voltage of a backlight in accordance with the characteristics of the video signals supplied to the display device that incorporates the backlight.
The technique disclosed in Jpn. Pat. Appln. KOAKI Publication 2004-110050 lies in using is to use the average picture level (APL) and the measured histogram distribution. First, a luminance-level region, e.g., a dark region, is designated and measured. Then, it is determined whether the histogram distribution measured exceeds the threshold value for the luminance-level region designated. If the histogram distribution does, the region is regarded as a dark region, regardless of the APL determined.
The technique disclosed in Jpn. Pat. Appln. KOAKI Publication 2005-148709 resides in measuring the histogram distribution from a video signal. The most frequent value F, i.e., the gradation existing more frequently in one image frame than any other gradation, is obtained from the distribution. In the backlight control unit, a plurality of regions have been set for gradations 0 to 255, each defined by eight bits. Those of the regions, which are pertinent to the value F, are examined. The backlight control unit outputs the control data set for the regions pertinent to the value F. The control data controls the amount of light the backlight emits.
In the conventional techniques, the regions for which the histogram distribution measured is applied are limited to those that comprise specific luminance levels. Further, the average picture level and the histogram distribution measured are used in combination, to control the backlight.
In the conventional image display devices, however, sufficient measures are not taken to reduce power consumption. If data representing the average picture level is used, the display screen may be frequently controlled brightness. In such a case, the image displayed will degrade in quality.